Pressure gauge

ABSTRACT

A retrofittable pressure gauge for a spray gun includes a pressure sensor, located in a pressure chamber, a power source and a display unit, all enclosed within a case, a passageway providing fluid communication between the pressure chamber and a pressurised interior space of the spray gun when the gauge is mounted on the spray gun and a mounting fixture configured for mounting the pressure gauge at a spreader valve or a trigger operated fluid valve of the spray gun.

BACKGROUND OF THE INVENTION

Traditionally the compressed air inlet pressure of a compressed airspray gun is adjusted to suit a particular compressed air spray gun aircap or spraying medium by a compressed air pressure reducing valve andair pressure gauge mounted upstream of the compressed air inlet, mountedeither adjacent to the source of the compressed air near a spray boothor mounted pendant of the handle of the compressed air spray gun. Oneproblem associated with the aforementioned arrangements of air pressuregauge and compressed air spray gun is that the location of the airpressure gauge is such that the air pressure upstream of the compressedair pressure reducing valve is measured. It is preferable to measure theair pressure downstream of the compressed air pressure reducing valveand all other compressed air valves which can affect a variable pressurereduction and lead to inconsistent compressed air pressure reaching theair cap.

It is possible for a compressed air inlet pressure reducing valve to beincluded as part of the spray gun thus eliminating the need for aseparate compressed air pressure reducing valve, however a problem withthis arrangement is that there is at present no means of displaying thecompressed air pressure, and if a separate compressed air pressurereducing valve is used upstream with an air pressure gauge, the pressurereducing valve is downstream and if not fully open can cause thepressure setting to vary.

A solution to the aforementioned problem is provided by EP 526 525 whichdiscloses a compressed air spray gun comprising a miniature digitalelectronic air pressure gauge located down stream of the compressed airinlet pressure reducing valve for measuring the air pressure at the aircap and downstream of the trigger operated shut off valve. As suchcompressed air spray guns are often used in hazardous areas, the airpressure gauge comprises a sealed unit with an aperture leading to apressure sensitive transducer connected to an amplifier. The signal fromthe amplifier is input to an analogue digital converter and thence to adecoder/driver and display. On exhaustion of the cell powering theamplifier, the sealed unit is discarded and replaced with a new sealedunit.

The miniature digital electronic air pressure gauge of EP 526 525 isthreaded into a threaded aperture bored into the compressed air spraygun. The air pressure gauge is arranged to be in communication with abore leading to a region downstream of the compressed air inlet pressurereducing valve and downstream of the trigger operated shut off valve.

A similar solution is offered by the ITW OMX-610 spray gun except thethreaded aperture receives an analogue pressure gauge and communicateswith a region downstream of the compressed air inlet pressure reducingvalve but upstream of the trigger operated shut off valve.

DE 100 31 857 A and DE 100 31 858 A disclose spray guns comprisingpressure gauges which are integrated into respectively the handle of thespray gun and the compressed air inlet valve. Embodiments are disclosedwith the compressed air pressure being measured upstream of thecompressed air inlet, between the compressed air inlet pressure reducingvalve and the trigger operated shut off valve and downstream of both thecompressed air inlet pressure reducing valve and the trigger operatedshut off valve.

Given the foregoing, a need has been identified to provide an airpressure gauge which can be retrofitted to a wide range of existingcompressed air spray guns without the need to modify the compressed airspray gun body and thereby measure the air pressure downstream of acompressed air inlet pressure reducing valve and trigger operated shutoff valve where the mounting of the gauge does not affect the ease ofuse of the existing spray gun controls.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a retrofittable pressure gaugefor a spray gun comprising a pressure sensor, located in a pressurechamber, a power source and a display unit, all enclosed within a case,a passageway providing fluid communication between the pressure chamberand a pressurised interior space of a spray gun when the gauge ismounted on a spray gun and a means for attachment to a spray gun,wherein the means for attachment comprises a mounting fixture configuredfor mounting the pressure gauge at a spreader valve or a triggeroperated fluid valve of a spray gun. The mounting fixture can comprise amounting fixture bore through which at least a part of the spreadervalve or trigger operated fluid valve can pass thereby mounting thepressure gauge at the spreader valve or the trigger operated fluidvalve. The passageway can provide fluid communication between thepressure chamber and the mounting fixture bore. Furthermore the mountingfixture bore and the part of the spreader valve or trigger operatedfluid valve which can pass through the mounting fixture bore can definean annular space in communication with the passageway.

The pressure gauge can comprise an analogue or digital pressure gauge.The pressure gauge optionally comprises a movement detector switch whichactivates the pressure sensor only when the pressure gauge is subject tomovement.

In one preferred embodiment, the pressure sensor comprises asemiconductor strain device. Typically the display unit is a liquidcrystal display. The pressure gauge preferably further comprises aprinted circuit board mounting a microprocessor.

In a second aspect of the invention, a kit is provided comprising thepressure gauge hereinbefore described and a spreader valve bushing or atrigger operated fluid valve bushing, the spreader valve bushing ortrigger operated fluid valve bushing comprising a channel, insubstantial alignment with the longitudinal axis of the spreader valvebushing or trigger operated fluid valve bushing, which, when thespreader valve bushing or trigger operated fluid valve bushing is fittedinto a spray gun, allows fluid communication between the pressurisedinterior space and the annular space.

In a third aspect of the invention, a spray gun is provided comprisingthe pressure gauge hereinbefore described.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention will now be described withreference to the figures in which:

FIG. 1 shows a side view of a partly sectioned compressed air spray guncomprising a digital electronic air pressure gauge according to theinvention attached to the compressed air spray gun at a spreader valve;

FIG. 2 shows an exploded view of a digital electronic air pressure gaugeaccording to the invention;

FIGS. 3 a–3 d show respectively a view on A—A indicated in FIG. 3 d, aview on B—B indicated on FIG. 3 c, a front view and a side view facingleft of a case of a digital electronic air pressure gauge according tothe invention;

FIG. 4 shows a part sectional view of part of a compressed air spray gunillustrating a digital electronic air pressure gauge according to theinvention mounted on a compressed air spray gun at a spreader valve;

FIG. 5 a–5 b show respectively a side view of a sectioned and a view onA—A indicated in FIG. 5 a of part of a movement detector switch of adigital electronic air pressure gauge according to the invention;

FIGS. 6 a–6 b show respectively a side view of a partly sectionedcompressed air spray gun and a part side view of a sectioned compressedair spray gun illustrating a digital electronic air pressure gaugeaccording to the invention mounted on a compressed air spray gun at atrigger operated fluid valve; and

FIG. 7 shows a side cross-section of a compressed air spray guncomprising concentric trigger operated shut off and trigger operatedfluid valve.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a spray gun comprising a gun body 101, defining an interiorspace 102 through which compressed air can be passed, having an integralhandle 111 depending from adjacent a first end of the gun body 116 andan air cap 112 secured to an opposing second end of the gun body 117 bya retaining ring 113.

A trigger 105 is secured to the gun body 101 by a screw 114 to pivottowards the handle 111 when manually squeezed to turn on the spray gun.A fluid reservoir 115 is reversibly attached to a fluid inlet 108protruding from an upper part of the gun body 101. A compressed air hose(not shown) is reversibly attached to a compressed air inlet 104 locatedon an end of the handle 111 remote from the gun body 101.

The volume of compressed air entering the spray gun is controlled by acompressed air inlet pressure reducing valve 110 operating within thehandle 111 of the spray gun. During use, as the trigger 105 is manuallysqueezed, compressed air from the compressed air hose (not shown) passesup through the handle 111 and through a trigger operated shut off valve106, located in the first end of the gun body 116, into the interiorspace 102 of the gun body 101. The interior space 102 communicatesseparately with atomisation air outlets (not shown) and spray patternshaping air outlets (not shown) in the air cap 112 by respectively, anatomising air passage (not shown) and a spray pattern shaping airpassage 118. During use, the volume of compressed air in the interiorspace 102 which passes through the spray pattern shaping air passage 118to the spray pattern shaping air outlets (not shown) is controlled by aspreader valve 103, also located in the first end of the gun body 116.

On manually squeezing the trigger 105, a trigger operated fluid valve107 is also opened permitting fluid, such as a paint, to pass from thefluid reservoir 115 through the fluid inlet 108 into a fluid passage(not shown) within the gun body 101 and out through a fluid dischargeorifice (not shown) as a stream. Compressed air exiting the atomisingair outlets atomises the fluid into particles as the stream of fluidleaves the fluid discharge orifice (not shown) producing a spray ofatomised fluid particles. Compressed air exiting the spray patternshaping air outlets (not shown) controls the shape of the said spray ofatomised fluid droplets.

An adjustable threaded end cap 119 sited at the first end of the gunbody 116 regulates the maximum volume of fluid which can pass throughthe fluid discharge orifice (not shown) by acting as a stop on thedegree to which the trigger operated fluid valve 107 can be opened.

A pressure gauge 109 is secured to the first end of the gun body 116 bythe spreader valve 103 thereby to measure the air pressure downstream ofthe compressed air inlet pressure reducing valve 110.

The pressure gauge 109 comprises a case 201, as shown in FIG. 2,including a recess 202, set into a side wall of the case 201, for aprinted circuit board (PCB) 203 mounting a single chip microprocessor.The PCB 203 is operatively linked to a two digit, seven segment liquidcrystal display (LCD) 204 by a contact strip 205. A cover 206 isprovided for the recess 202 comprising a window 207 through which theLCD 204 may be viewed.

In the floor of the recess 202 are set a first well 208 for a pressuretransducer 209, and a second well 210 for a battery 211, the pressuretransducer 209 and the battery 211 being independently operativelylinked to the PCB 203. The pressure transducer 209 is controlled by thePCB mounted microprocessor. The first well 208 and the pressuretransducer 209 together define a pressure chamber (not shown).

The case 201 additionally comprises a mounting fixture 212 pendant froma lower surface of the case 201, the mounting fixture 212 comprising amounting fixture bore 213 substantially in alignment with the LCD 204axis of view. The case 201 also comprises an air passage 301, as shownin FIGS. 3 a and 3 b, providing communication between the first well 208and the mounting fixture bore 213.

FIG. 4 shows a detail of the mounting of the pressure gauge 109 onto thefirst end 116 of the gun body 101 by the spreader valve 103. Thespreader valve 103 comprises a male thread 401 on part of an exteriorsurface of a shank 402, the shank 402 terminating at a first end 403with a tip 404, which can progressively block the entrance to the spraypattern shaping air passage 118, and at a second end 405 by a knurlednut 406 rotation of which rotates the shank 402 causing translation ofthe tip 404 thereby blocking the said entrance. The male thread 401 arelocated approximately between the knurled nut 406 and a point midwaybetween the knurled nut 406 and the tip 404.

The spreader valve 103 additionally comprises a spreader valve bushing407, comprising a female thread (not shown) located on an inside surface414 of the spreader valve bushing 407, in which the shank 402 isretained by interaction between the female thread (not shown) and themale thread 401. The spreader valve bushing 407 is threaded into athreaded aperture 408 bored into the first end 116 of the gun body 101terminating at the interior space 102. The spreader valve bushing 407 isprovided with an annular channel 409 formed on the inside surface 414substantially in axial alignment with the longitudinal axis of thespreader valve bushing 407. A first end 410 of the annular channel 409terminates at a first end 411 of the spreader valve bushing 407 locatedwithin the aperture 408 and a second end 412 of the annular channel 409terminates approximately mid-way between the first end 411 and a secondend 413 of the spreader valve bushing 407. The second end 412 of theannular channel 409 communicates with an opening 416 through thespreader valve bushing 407 connecting the inside surface 414 and outsidesurface 415 of the spreader valve bushing 407. The female thread (notshown) lies approximately between the second end 413 of the spreadervalve bushing 407 and a point midway between the first end 411 andsecond end 413 of the spreader valve bushing 407.

The pressure gauge 109 is mounted onto the spray gun by passing thespreader valve bushing 407 through the mounting fixture bore 213 of thepressure gauge 109 and threading the spreader valve bushing 407 into thethreaded aperture 408. The pressure gauge 109 is thereby clamped betweena first surface defined by the exterior surface of the gun body adjacentthe aperture 408 and a second surface defined by a spreader valvebushing shoulder 417. The spreader valve bushing 407 and shank 402 ofthe spreader valve 103 are adapted to be longer in the present inventionthan typically used when not mounting the pressure gauge 109 of theinvention. In another embodiment, the spreader valve bushing 407 andshank 402 of the spreader valve 103 remain unchanged in length in thepresent invention. In the latter case, only the spreader valve bushing407 need be adapted and modified to mount the pressure gauge 109 to thespray gun by, for example, the provision of a removable ring locatedbetween the knurled nut 406 and the second end 413 of the spreader valvebushing 407. The pressure gauge 109 is mounted on the spray gun in placeof the removable ring.

Furthermore, the inside diameter of the mounting fixture bore 213 islarger than the outside diameter of the spreader valve bushing 407thereby creating a spreader valve annular space 418 between the opposingsurfaces defined by the inside surface of the mounting fixture bore andthe outside surface of the bushing 415.

On mounting the pressure gauge as described hereinabove, compressed airin the interior space 102 can pass through the channels 409 formed onthe inside surface 414 of the spreader valve bushing 407 and through theopenings 416 and thence through the air passage 301, via the spreadervalve annular space 418, into the first well 208 of the pressure gauge109 wherein resides the pressure transducer 209.

The pressure transducer 209 comprises a semiconductor strain gaugedevice bonded to a glass substrate mounted on a ceramic base whosecharacteristics change when subjected to a physical strain induced by,in this case, compressed air. The pressure transducer 209 generates asignal which is input to the PCB mounted microprocessor which includesan analogue digital convertor. The output of the PCB mountedmicroprocessor is fed to the LCD 204 which indicates the measured airpressure.

The pressure gauge 109 operates in the following manner. The pressuregauge 109 can be in any one of two operational states, namely an activemode and an idle mode. In the active mode, the pressure gauge 109actively measures and displays the current compressed air pressure.Should the compressed air pressure remain at zero for longer than apredetermined period, the pressure gauge 109 will then automaticallyswitch to the idle mode, in order to conserve battery 211 life, at whichpoint the LCD 204 will go blank. In the idle mode, the PCB mountedmicroprocessor will activate the pressure transducer 209 for a shortperiod at regular intervals. If a compressed air pressure aboveatmospheric pressure is detected, the pressure gauge 109 willimmediately switch to the active mode again.

To further increase the life of the battery 211, the pressure gauge 109can additionally comprise a movement detector (not shown in FIG. 2)which signals the microprocessor to de-energise the pressure transducer209 after the spray gun has been stationary for a predetermined periodthereby conserving battery life. When the spray gun is moved, themovement detector (not shown) detects this movement and signals themicroprocessor to energise the pressure transducer 209.

A suitable movement detector switch, shown in FIGS. 5 a and 5 b,comprises a metal ball 501 trapped in a slot 502 set into the surface ofa metal housing 503, an open side of the slot 502 covered by the PCB203. FIG. 5 a shows that the PCB 203 comprises a first through contacthole 504 permitting electrical contact between a first copper track 505and a first copper contact pad 506. In FIG. 5 b a second copper contactpad 507 is shown adjacent the first copper contact pad 506, the secondcopper contact pad 507 in electrical contact with a second copper track(not shown) via a second through contact hole (not shown). The slot 502is shown in FIG. 5A angled slightly downwardly to ensure the metal ball501 remains in contact with at least one of the first or second coppercontact pads 506, 507 when the spray gun is held upright with thetrigger operated fluid valve 107 in a horizontal position.

The metal ball 501 completes a first and second electric circuit betweenrespectively the first 506 or second 507 copper contact pad and themetal housing 503 thereby producing input signals to the single chipmicroprocessor mounted on the PCB 203. The metal ball 501 can eithercomplete the first electric circuit between the first copper contact pad506 and the metal housing 503, or the second electric circuit betweenthe second copper contact pad 507 and the metal housing 503 depending onthe location within the slot 502 of the metal ball 501. When the spraygun is being used, the metal ball 501 can move within the slot 502successively completing the first and second electric circuits.Completion of each electric circuit generates different input signalswhich the single chip microprocessor recognises. When the microprocessorreceives successive alternating inputs from each of the two circuits,the microprocessor energises the pressure transducer 209. When thealternating input signals cease for a predetermined time, due to thespray gun being motionless, the microprocessor de-energises the pressuretransducer 209.

As shown in FIGS. 6 a and 6 b, the pressure gauge 109 can be mounted ona compressed air spray gun in a similar manner at the trigger operatedfluid valve 107. The trigger operated fluid valve 107 comprises a needle601, including a first needle end (not shown) terminating at an apex anda second needle end 602, and a first shoulder (not shown) locatedapproximately mid-way between the first (not shown) and second 602needle ends of the needle. The trigger operated fluid valve 107 furthercomprises a fluid valve bushing 603, which fits over the needle 601,including first 604 and second 605 fluid valve bushing ends. The secondfluid valve bushing end 605 is capped with the adjustable threaded endcap 119. The first fluid valve bushing end 604 terminates adjacent thetrigger 105. Around a portion of the needle 601 bounded by theadjustable threaded end cap 119 and a second shoulder 606 on the needle601 lies a spring 607. The fluid valve bushing 603 extends through thegun body 101

When the spring 607 is in a relaxed state, the first needle end (notshown) blocks the fluid discharge orifice (not shown). On manuallysqueezing the trigger 105, a face (not shown) on the trigger 105impinges on the first shoulder (not shown) forcing the needle 601 tomove away from and thereby to progressively unblock the fluid dischargeorifice (not shown) compressing the spring 607.

FIG. 6 b shows that the fluid valve bushing 603 is provided with a fluidvalve bushing channel 608 on the exterior surface of the fluid valvebushing 609 permitting communication between the interior space 102 anda fluid valve annular space 610 formed in identical fashion as describedhereinabove for the spreader valve 103.

The pressure gauge 109 is mounted onto the spray gun by passing thefluid valve bushing 603 through the mounting fixture bore 213 andthreading the fluid valve bushing 603 into a threaded bore 611 locatedin the first end of the gun body 116 below the spreader valve 103. Thepressure gauge 109 is thereby clamped between a first surface defined bythe exterior surface of the gun body adjacent the threaded bore 611 anda second surface defined by a fluid valve bushing shoulder 612. Thefluid valve bushing 603 and needle 601 are adapted to be longer in thepresent invention than typically used when not mounting the pressuregauge 109 of the invention. In another embodiment, the fluid valvebushing 603 and needle 601 remain unchanged in length in the presentinvention. In the latter case, only the fluid valve bushing 603 need beadapted and modified to mount the pressure gauge 109 to the spray gunby, for example, the provision of a removable ring located between theknurled nut 406 and the second end of the spreader valve bushing 413.The pressure gauge 109 is mounted on the spray gun in place of theremovable ring.

The pressure gauge 109 can also be mounted, in a similar manner asdescribed hereinabove, on spray guns of alternative design and layout.Such spray guns include that illustrated in DE 100 31 858 A where thelongitudinal axis of the spreader valve is substantially orthogonal tothe plane of the spray gun. The trigger operated fluid valve of thespray gun is in a similar location to that shown in FIG. 1. Thecompressed air inlet pressure reducing valve is, however, sited on thegun body between the trigger operated fluid valve and a handle pendantfrom one end of the gun body. Another design of gun on which thepressure gauge of the invention can be mounted is that shown in FIG. 7where the trigger operated shut off valve and trigger operated fluidvalve are concentric. A further spray gun layout suitable foraccommodating the pressure gauge of the invention is a spray guncombining concentric trigger operated compressed air and triggeroperated fluid valves with a spreader valve whose longitudinal axis issubstantially orthogonal to the plane of the spray gun.

It will be appreciated that various modifications and changes may bemade to the above described preferred embodiments of the inventionwithout departing from the invention as set out in the following claims.

1. A kit, comprising a retrofittable pressure gauge and a valve bushing of a spreader valve or a trigger operated fluid valve of a spray gun; wherein said pressure gauge comprises: a case having a pressure chamber; a pressure sensor located in the pressure chamber; and a mounting fixture attached to the case and having a through bore for mounting the pressure gauge at the spreader valve or the trigger operated fluid valve of the spray gun, wherein the mounting fixture further comprises a passageway other than said through bore, said passageway providing fluid communication between the pressure chamber and the through bore; wherein said valve bushing has a first section sized to be passable through said through bore and a second section sized to be not passable through said through bore, said second section defining a shoulder engageable with one of two opposite end surfaces of said mounting fixture when the first section of said valve bushing is received in the through bore; wherein said through bore extends between said opposite end surfaces and has an opening on each of said opposite end surfaces, and an entire axial extent of said through bore between said opposite end surfaces is smaller than an axial dimension of said first section of said valve bushing; wherein, when said first section is inserted into said through bore until the shoulder defined by said second section comes to rest on said one of said opposite end surfaces of the mounting fixture, said first section extends entirely through said through bore and projects beyond the other one of said opposite end surfaces; wherein, when the first section of said valve bushing is received in the through bore, the first section is radially spaced from an inner wall of said through bore, thereby defining between said bushing and said through bore an annular channel which, when the valve bushing is fitted into the spray gun, allows fluid communication between a pressurized interior space of the spray gun and the passageway, and hence, the pressure chamber; and wherein said valve bushing is hollow and further includes a tubular sidewall surrounding an inner space of said valve bushing; and a through hole in a middle region of said tubular sidewall, thereby providing fluid communication between the inner space of said valve bushing and the pressure chamber via said passageway and said annular channel.
 2. The kit according to claim 1, further comprising a shank receivable within and extendable entirely though said inner space of said valve bushing.
 3. A kit, comprising a retrofittable pressure gauge and a valve bushing of a spreader valve or a trigger operated fluid valve of a spray gun; wherein said pressure gauge comprises: a case having a pressure chamber; a pressure sensor located in the pressure chamber; and a mounting fixture attached to the case and having a though bore for mounting the pressure gauge at the spreader valve or the trigger operated fluid valve of the spray gun, wherein the mounting fixture further comprises a passageway other than said though bore, said passageway providing fluid communication between the pressure chamber and the through bore; wherein said valve bushing has a first section sized to be passable through said through bore and a second section sized to be not passable through said through bore, said second section defining a shoulder engageable with one of two opposite end surfaces of said mounting fixture when the first section of said valve bushing is received in the through bore; wherein said though bore extends between said opposite end surfaces and has an opening on each of said opposite end surfaces, and an entire axial extent of said though bore between said opposite end surfaces is smaller than an axial dimension of said first section of said valve bushing; wherein, when said first section is inserted into said through bore until the shoulder defined by said second section comes to rest on said one of said opposite end surfaces of the mounting fixture, said first section extends entirely through said through bore and projects beyond the other one of said opposite end surfaces; and wherein said mounting fixture further comprises two side surfaces connecting the opposite end surfaces and extending from opposite sides of said case toward each other and away from said case, wherein said though bore is formed at a corner portion of said mounting fixture where said side surfaces meet.
 4. A kit, comprising a retrofittable pressure gauge and a valve bushing of a spreader valve or a trigger operated fluid valve of a spray gun; wherein said pressure gauge comprises: a case having a pressure chamber; a pressure sensor located in the pressure chamber; and a mounting fixture attached to the case and having a though bore for mounting the pressure gauge at the spreader valve or the trigger operated fluid valve of the spray gun, wherein the mounting fixture further comprises a passageway other than said though bore, said passageway providing fluid communication between the pressure chamber and the through bore; wherein said valve bushing has a first section sized to be passable through said through bore and a second section sized to be not passable through said through bore, said second section defining a shoulder engageable with one of two opposite end surfaces of said mounting fixture when the first section of said valve bushing is received in the through bore; wherein said though bore extends between said opposite end surfaces and has an opening on each of said opposite end surfaces, and an entire axial extent of said though bore between said opposite end surfaces is smaller than an axial dimension of said first section of said valve bushing; wherein, when said first section is inserted into said through bore until the shoulder defined by said second section comes to rest on said one of said opposite end surfaces of the mounting fixture, said first section extends entirely through said through bore and projects beyond the other one of said opposite end surfaces; and wherein said opposite end surfaces are parallel and said though bore has a constant diameter thoughout the entire axial extent thereof.
 5. A spray gun, comprising: a valve including a bushing partially received in a bore formed at an end of the gun and a shank received within said bushing; and a retrofittable pressure gauge mounted at said valve, said pressure gauge comprising: a case having a pressure chamber; a pressure sensor located in the pressure chamber; and a mounting fixture attached to the case and having a through bore, wherein the mounting fixture further comprises a passageway other than said through bore, said passageway providing fluid communication between the pressure chamber and the though bore of said mounting fixture; wherein said bushing has a first section passes though the though bore of said mounting fixture and a second section which is not passable though the though bore of said mounting fixture, said mounting fixture being detachably clamped between said second section of the bushing and said end of the gun; wherein said mounting fixture comprises opposite surfaces, said though bore extends between said opposite surfaces and has an opening on each of said opposite surfaces, and an entire axial extent of said through bore between said opposite surfaces is smaller than an entire dimension of said case as measured in an axial direction of said through bore; and wherein said opposite surfaces are parallel and said though bore has a constant diameter throughout the entire axial extent thereof.
 6. A spray gun, comprising: a valve including a bushing partially received in a bore formed at an end of the gun and a shank received within said bushing; and a retrofittable pressure gauge mounted at said valve, said pressure gauge comprising: a case having a pressure chamber; a pressure sensor located in the pressure chamber; and a mounting fixture attached to the case and having a through bore, wherein the mounting fixture further comprises a passageway other than said through bore, said passageway providing fluid communication between the pressure chamber and the though bore of said mounting fixture; wherein said bushing has a first section passes though the though bore of said mounting fixture and a second section which is not passable though the though bore of said mounting fixture, said mounting fixture being detachably clamped between said second section of the bushing and said end of the gun; and wherein said mounting fixture is detachably clamped between said second section of the bushing and said end of the gun by threaded engagement between said valve bushing and the bore formed in said end of said gun.
 7. A spray gun, comprising: a valve including a bushing partially received in a bore formed at an end of the gun and a shank received within said bushing; and a retrofittable pressure gauge mounted at said valve, said pressure gauge comprising: a case having a pressure chamber; a pressure sensor located in the pressure chamber; and a mounting fixture attached to the case and having a through bore, wherein the mounting fixture further comprises a passageway other than said through bore, said passageway providing fluid communication between the pressure chamber and the though bore of said mounting fixture; wherein said bushing has a first section passes though the through bore of said mounting fixture and a second section which is not passable though the though bore of said mounting fixture, said mounting fixture being detachably clamped between said second section of the bushing and said end of the gun; and said mounting fixture comprises opposite end surfaces, said through bore extends between said opposite end surfaces and has an opening on each of said opposite end surfaces, said bushing extends entirely though said though bore, and said shank extends entirely though both said bushing and said through bore. 